Spin-glass ordering in the diluted magnetic semiconductor Zn1-xMnxTe

Magnetic measurements on the spin-glass behavior in the bulk II-VI diluted magnetic semiconductor (DMS) ZnMnTe were made on two crystals of concentrations x = 0.43 and 0.55 taken from the same boule. Magnetization and density functional theory studies have shown paramagnetic behavior in both samples between 30 and 400 K. Below 30 K, there is a prominent peak at Tc = 15 and 23.6 K for concentrations x = 0.43 and 0.55, respectively. The splitting of the field cooled (FC) and zero field cooled (ZFC) data below this peak is indicative of a transition to a spin-glass state at low temperature for semiconductors. Therefore, through the p− and d− orbits hybridization a magnetic exchange produces the spin-glass behavior seen in the DMS ZnMnTe

Raman study of the Verwey transition in Magnetite at high-pressure and low-temperature; effect of Al doping

We report high-pressure low-temperature Raman studies of the Verwey transition in pure and Al-doped magnetite (Fe_3O_4). The low temperature phase of magnetite displays a number of additional Raman modes that serve as transition markers. These transition markers allow one to investigate the effect of hydrostatic pressure on the Verwey transition temperature. Al-doped magnetite Fe_2.8Al_0.2O_4 (TV=116.5K) displays a nearly linear decrease of the transition temperature with an increase of pressure yielding dP/dT_V = -0.096 GPa/K. In contrast pure magnetite displays a significantly steeper slope of the PT equilibrium line with dP/dT_V = -0.18 GPa/K. The slope of the PT equilibrium lines is related to the changes of the molar entropy and molar volume at the transition. We compare our spectroscopic data with that obtained from the ambient pressure specific heat measurements and find a good agreement in the optimally doped magnetite. Our data indicates that Al doping leads to a smaller entropy change and larger volume expansion at the transition. Our data displays the trends that are consistent with the mean field model of the transition that assumes charge ordering in magnetite.Comment: 17 pages, 3 figure

Reduced Dimensionality Effects in Ferromagnetic Behavior in La1-xSrxMnO3

We study the magnetic properties of La1-xSrxMnO3 samples for concentrations x, 0 ≤ x ≤ 0.5. An analysis is done to accurately determine the transition temperature or critical temperature. Magnetic phase diagrams showing the various concentrations at different temperatures will be determined for our thin films. Using the phase diagrams for both bulk and thin film materials can show how reducing the dimensionality from the third dimension to approaching the second-dimension affects the phase diagram

Self-referencable frequency comb from a 170-fs, 1.5-μm solid-state laser oscillator

We report measurement of the first carrier-envelope offset (CEO) frequency signal from a spectrally broadened ultrafast solid-state laser oscillator operating in the 1.5μm spectral region. The f-to-2f CEO frequency beat signal is 49 dB above the noise floor (100-kHz resolution bandwidth) and the free-running linewidth of 3.6 kHz is significantly better than typically obtained by ultrafast fiber laser systems. We used a SESAM mode-locked Er:Yb:glass laser generating 170-fs pulses at a 75MHz pulse repetition rate with 110-mW average power. It is pumped by one standard telecom-grade 980-nm diode consuming less than 1.5W of electrical power. Without any further pulse compression and amplification, a coherent octave-spanning frequency comb is generated in a polarization-maintaining highly-nonlinear fiber (PM-HNLF). The fiber length was optimized to yield a strong CEO frequency beat signal between the outer Raman soliton and the spectral peak of the dispersive wave within the supercontinuum. The polarization-maintaining property of the supercontinuum fiber was crucial; comparable octave-spanning supercontinua from two non-PM fibers showed higher intensity noise and poor coherence. Astable CEO-beat was observed even with pulse durations above 200fs. Achieving a strong CEO frequency signal from relatively long pulses with moderate power levels substantially relaxes the demands on the driving laser, which is particularly important for novel gigahertz diode-pumped solid-state and semiconductor laser

Picosecond diode-pumped 1.5 μm Er,Yb:glass lasers operating at10-100 GHz repetition rate

Stable ultrafast laser sources at multi-GHz repetition rates are important for various application areas, such as optical sampling, frequency comb metrology, or advanced high-speed return-to-zero telecom systems. We review SESAM-mode-locked Er,Yb:glass lasers operating in the 1.5μm spectral region at multi-GHz repetition rates, discussing the key improvements that have enabled increasing the repetition rate up to 100GHz. We also present further improved results with shorter pulse durations from a 100GHz Er,Yb:glass laser. With an improved SESAM design we achieved 1.1ps pulses with up to 30mW average output power. Moreover, we discuss for the first time the importance of beam quality deteriorations arising from frequency-degenerate higher order spatial modes in such laser

Spin glass or random anisotropy?: The origin of magnetically glassy behavior in nanostructured GdAl\u3csub\u3e2\u3c/sub\u3e

Initially crystalline GdAl2 was mechanically milled for long times to produce a highly chemically disordered phase with approximately 8-nm grains. Analysis of dc magnetization measurements using an Arrott plot and the approach to saturation suggest the presence of significant random anisotropy. ac susceptibility measurements showed that the shift in the peak temperature with frequency usually seen in magnetically glassy and superparamagnetic systems was virtually undetectable in the 10–1000-Hz frequency range. Based on these results, we believe that this material represents an interacting system with random anisotropy, where the anisotropy is the result of surface and interface asymmetries. ©2005 American Institute of Physic

Magnetic Properties of MBE Grown La1/3Y1/3Sr1/3MnO3 Thin Films and Superlattices

We have investigated the magnetic properties of thin films related to the standard CMR system La2/3Sr1/3MnO3 where Y substituted for 50% of the La atoms. These La1/3Y1/3Sr1/3MnO3 films were grown as a random alloy where La, Y, and Sr atoms randomly occupied the A-site or as a superlattice where each unit-cell-thick layer stacked along the crystallographic (001) direction contained only one of the atoms La, Y, and Sr occupying the A-site. One of the key magnetic features of La2/3Sr1/3MnO3 is a prominent ferromagnetic transition near 350 K. We find the substitution of La with Y suppresses this ferromagnetic transition in both the random alloy and the superlattice samples. In the superlattice sample we find a magnetic transition that is coincident with a metal-to-insulator transition we observe in electronic transport. In the random alloy sample, we see a similar magnetic transition but at lower temperatures where we find the sample is too insulating to measure electronic transport. We will compare our measurements on these La1/3Y1/3Sr1/3MnO3 samples with CMR thin films of La2/3Sr1/3MnO3

Magnetic Properties of MBE Grown La0.6Sr0.4MnO3 Thin Films

Honorable Mention Winner This project investigates the magnetic properties of a La1-xSrxMnO3 (x = 0.40) sample of high quality. This sample was grown one atomic layer at a time by Prof. Warusawithana using UNF’s Molecular Beam Epitaxy (MBE) machine. These magnetic properties are investigated over a range of temperatures from 5 to 400 K in fields up to 7 T. We make use of the techniques to analyze the sample to determine to a high degree of precision the critical temperature of the sample, we determined it to be 252 K. We further identified the saturated magnetization, remnant magnetization, and coercive field at 5 K to be 0.00733 emu/g, 0.00563 emu/g and 0.0090 T respectivel

Theory of Diluted Magnetic Semiconductor Ferromagnetism

We present a theory of carrier-induced ferromagnetism in diluted magnetic semiconductors (III_{1-x} Mn_x V) which allows for arbitrary itinerant-carrier spin polarization and dynamic correlations. Both ingredients are essential in identifying the system's elementary excitations and describing their properties. We find a branch of collective modes, in addition to the spin waves and Stoner continuum which occur in metallic ferromagnets, and predict that the low-temperature spin stiffness is independent of the strength of the exchange coupling between magnetic ions and itinerant carriers. We discuss the temperature dependence of the magnetization and the heat capacity